Austin , Texas -LRB- CNN -RRB- -- The man who flew an airplane into a building housing an Internal Revenue Service office may have replaced some of its seats with a drum of fuel to cause maximum damage , a law enforcement official familiar with the investigation said Friday .

The official , who would not speak on the record because it is an ongoing case , said investigators have determined that the Piper Cherokee PA-28 had several seats removed and that a fuel drum was missing from the airport from which Andrew Joseph `` Joe '' Stack III took off Thursday morning .

`` I think there is a good chance he might have put it on his plane , '' said the official , who cautioned that investigators were still working that lead and sifting through the crash site .

The single-engine plane has a fuel tank capacity of 38 gallons and is equipped with four seats , according to the Web site risingup.com .

The FBI said Friday it has taken the lead role in the investigation of Thursday 's crash into the 7-story building in northwest Austin that held offices for nearly 200 IRS workers .

`` You 're talking about a federal agency that was basically assaulted , '' Ralph Diaz , special agent in charge of the FBI 's San Antonio , Texas , field office , told reporters .

Two people were killed and two others were hospitalized , federal officials said .

Though the remains found in the IRS building have been identified , their identities will not be revealed until after a forensic examination , Diaz said Friday .

`` One may be Mr. Stack and our fear is certainly the other would be either someone who was visiting or someone who was employed in the building , '' he said .

Emergency services chief Ernie Rodriguez said one of the injured was treated and released ; the other remained hospitalized .

City of Austin Fire Chief Rhoda Mae Kerr credited the building 's fire protection , fast work by the Fire Department and the fact that the employees had practiced exiting the building in case of an emergency for the low loss of life and injuries . `` It truly worked , '' she said .

Austin attack stuns community

Rodriguez said units arrived on the scene within five minutes of the first 911 call prepared to manage hundreds of injured people . `` We found only two persons , '' he said .

`` When you look at the fire , when you look at the evidence , it 's hard to say that we were lucky , but we were , '' he said . `` The big thing to remember is that yesterday , in the midst of this event , God 's grace was upon us . ''

Austin Police Chief Art Acevedo credited fast thinking by some people who were inside the building as the plane approached for the fact that few people were hurt . `` Some folks saw it coming and sounded the alarm yesterday , and some folks started running away from that side of the building , '' he said .

Stack 's wife , Sheryl Stack , expressed her `` sincere sympathy to the victims and their families '' Friday .

`` Words can not adequately express the sorrow or the sympathy I feel for everyone affected by this unimaginable tragedy , '' she said in a statement read by a family friend , Rayford Walker .

Authorities say Stack also torched his $ 230,000 home in Austin on Thursday morning before embarking on his fatal flight .

Police said Sheryl Stack spent Wednesday night in an Austin-area hotel but did not say why . Acevedo said police had received no calls of domestic violence from the house . The only calls to police were made a couple of years ago and concerned barking dogs , he said .

A 3,000-word message on a Web site registered to Stack railed against the government , particularly the IRS .

Read the apparent suicide note -LRB- PDF -RRB-

`` I saw it written once that the definition of insanity is repeating the same process over and over and expecting the outcome to suddenly be different , '' the online message says . `` I am finally ready to stop this insanity . Well , Mr. Big Brother IRS man , let 's try something different ; take my pound of flesh and sleep well . ''

Acevedo reiterated on Friday his assertion first made Thursday that the incident was not an act of terrorism . `` Part of our jobs in law enforcement is not to overreact and cause undue panic , '' he said . `` And with the information that we had , there was no need to alarm our colleagues around the country and community members by using the word ` terrorism . ' That is why definitely I did not use it yesterday and I 'm not using it today . ''

An IRS criminal investigator visited Stack 's accountant after Thursday 's events , leaving a business card on the CPA 's front door . The online message believed to have been written by Stack mentions accountant Bill Ross , saying it had become `` brutally evident that he was representing himself and not me . ''

Watch what Stack 's writing reveals about him

IRS investigator James L. Neff said that Ross is not under investigation ; officials just wanted to make sure he was OK . `` We did n't know if he was alive or dead , '' Neff said .

Ross , who did not return a call from CNN , was fine , the investigator said .

Friends and former colleagues said Friday they had no inkling of the rage apparently building inside Stack .

`` He hid that very well , '' said Billy Eli , in whose band Stack played bass until a few years ago . `` Obviously he was in some serious distress and had some real despair . I never saw that . ''

Neither did another former bandmate , Ric Furley .

Watch how bandmates remember Stack

`` I never saw him in a bad mood or speaking negatively about anything or anyone , '' Furley told CNN 's `` American Morning . ''

`` This has been such a shock because it was totally out of character with the Joe Stack I played with for three years , '' said Eli .

`` We liked him , '' said Furley . `` We liked him . ''

Share information about the crash and the pilot

An IRS spokesman said federal law prohibits the agency from releasing information about any interactions the agency may have had with Stack .

The IRS issued a statement Friday saying tax returns will not be delayed .

CNN 's David Mattingly , Ed Lavandera and Carol Cratty contributed to this report .

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NEW : Seats removed from plane , fuel drum missing from airport

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FBI takes lead role in investigation into Austin , Texas , crash

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Pilot 's wife expresses her `` sincere sympathy to the victims and their families ''

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Joseph Stack believed to have flown plane into IRS building in suicidal attack